Method for modifying the physical properties of aluminum casting alloys



United States Patent Ofitice 3,355,281 Patented Nov. 28, 1967 3,355,281METHOD FOR MODIFYING THE PHYSICAL PROP- ERTIES OF ALUMINUM CASTINGALLOYS Virgil L. Hansley and Fred K. Morgan, Cincinnati, Ohio,

assignors to National Distillers and Chemical Corporation, New York,N.Y., a corporation of Virginia No Drawing. Filed Apr. 22, 1964, Ser.No. 361,900

Claims. (Cl. 75-138) This invention relates to a method for enhancingthe physical properties of metals by the use of additive alloys.

More particularly, the invention pertains to the modification ofaluminum casting alloys with an admixture of alkali and alkaline earthmetals employed in the form of an aluminum-alkaline earth metal-alkalimetal master alloy. Another aspect of the present invention resides in anovel method for preparing the master alloy.

It is known in the art that aluminum-silicon alloys which have beentreated with an alkali metal, especially sodium, have grain structuresand mechanical properties which are superior to untreatedaluminum-silicon alloys. Metallic sodium, for example, has been usedcommercially to modify the grain structure of aluminum-silicon alloysand to improve the mechanical properties of castings made therefrom.U.S. Patent No. 1,410,461, while describing such a modification method,also recognizes that when metallic sodium is added to the molten castingalloy, the sodium burns resulting in objectionable smoke and fumes aswell as the loss of some of the sodium by volatilization. This patentsuggests that amount of sodium lost may be reduced by adding the sodiumin the form of an alloy with a heavy metal such as zinc. The problemsattendant upon the use of metallic sodium are further recognized in US.Patent No. 1,657,389, which proposes the use of an alkaline earth metalsuch as calcium to achieve the desired modification of aluminum castingalloys. However, it has been found diflicult to add calcium in traceamounts to the aluminum alloys because of its high melting point andreactivity with both oxygen and nitrogen in the atmosphere. Thecomparatively high cost of pure calcium can also be detrimental in thecommercial use of such a modification method.

One object of the present invention is to provide a novel method formodifying the physical properties of metals or metal alloys such asaluminum-silicon alloys.

Another object of the present invention is to provide a novel method formodifying the processing characteristics as well as the grain structureand mechanical properties of aluminum-silicon alloys which avoids thedifliculties and drawbacks of the prior art methods.

A further object of the present invention is to provide an additivealloy comprising aluminum, an alkaline earth metal and an alkali metalwhich can be employed to modify the physical properties and handlingproperties of metals such as aluminum-silicon alloys.

A still further object of the present invention is to provide a new andimproved method for preparing the additive or master alloy.

These and other objects of the present invention will become readilyapparent from the ensuing description and illustrative embodiments.

In accordance with the present invention it has now been found that thephysical properties of metals such as aluminum-silicon alloys can bereadily improved by the addition thereto of a minor amount of a masteralloy of aluminum with an alkaline earth metal and an alkali metal. Ingeneral, the master alloy contains about 0.5 to 20% by weight of thealkaline earth metal, about 0.01 to 1.0% by weight of the alkali metal,and from about 79 to 99% by weight of aluminum. The preferred masteralloy contains about 4 to by weight of the alkaline earth specifically,

metal, about 0.1 to 0.5% by weight of the alkali metal, and from about89.5 to 95.9% by weight of aluminum.

The properties of the master alloy, in the 0.5 to 20% by weight alkalineearth metal range, are such that it can be cast into convenient barssuch as round bars having a diameter, for example, of about one andone-half inches. The alloy is also brittle enough to permit simplebreaking off of the amounts required in the particular treatingoperation. Moreover, the alloy is safe to handle, since it does notreact with water and contains sutiicient treating metals so that theamount needed is easily melted and dissolved by the superheat in themolten metal or alloy.

Only a relatively small quantity of the master alloy additive agent isrequired to effect the desired changes in the casting metals or alloys.It has been found, for ex ample, that the master alloy need only beadded to the aluminum-silicon alloy or other metal composition beingtreated in amount sufficient to yield from about 0.001 to about 0.10% byweight of the alkaline earth and alkali metals, based on the totalweight of the resulting composition. In accordance with the preferredmethod of operation, the resulting or final metal composition containsonly about 0.01 to about 0.05% by weight of total alkaline earth andalkali metals.

Although the alkaline earth metals employed in preparing the masteralloys of this invention may be selected from the group consisting ofmagnesium, calcium, strontium, barium and mixtures thereof; thepreferred alkaline earth metal has been found to be calcium. On theother hand, the preferred alkali metal is'sodium. It will be understood,however, that other alkali metals such as potassium, lithium, etc., andmixtures of alkali metals may also be employed, For purposes ofillustration, the invention will be described more specifically in termsof a master alloy comprising calcium, sodium and aluminum.

As previously indicated the master alloy of this invention can beemployed to modify the physical properties of a variety of metals andmetal alloys. The invention will be illustrated hereinafter with respectto the use of the master alloy in the modification of aluminum castingalloys containing silicon, magnesium or zinc. Nevertheless, other metalsand metal alloys such as, for example, iron and steel may be similarlytreated.

The use of the master alloy of this invention to elfect metalmodification has the further advantages of avoiding the addition ofcontaminating heavy metals and of not substantially altering the ratioof aluminum to alloying metal in a casting alloy such asaluminum-silicon. In general, aluminum-silicon casting alloys, forexample, contain about 3 to 15%, and preferably about 8 to 13% by weightof silicon. Thus, the inclusion of a non-silicon containing alloy in theadditive as a vehicle for the alkaline earth and alkali metals does notsubstantially alter the aluminum-silicon ratio of the final castingmetal. More it has been found that an aluminum-calciumsodium masteralloy carrying a total of 5% calcium and sodium can be added to aaluminum-10% silicon alloy to a level of about 0.01 to 0.05% calciumplus sodium with a resulting change only in the order of one part perhundred in the silicon content. That is, the silicon content would onlydrop from 10% to 9.9%, which is usually within the variability limitsfor these constituents in the alloy..

The master alloy of this invention, e.g. a calcium-sodium-aluminumalloy, may be prepared in any convenient manner known in the alloypreparation art. It can be formed, for example, by heating pure aluminummetal in a suitable vessel, e.g. gray cast iron or a graphite crucible,until molten and then rapidly adding calcium and sodium with stirring atabout 700 to 1000 C., and preferably at about 750 to 850 C., and holdingthe temperature in this range until uniform composition is obtained.

Percent by weight Calcium 18-22 Sodium 60-65 CaO, Na O, Inerts 13-22Broadly, however, impure mixtures containing from 9 to 25% calcium, 50to 75% sodium, and up to 25% inerts may be employed.

In general, the method of preparation comprises agitating a. meltcontaining the aluminum to be alloyed, the sodiumbalcium or sodiumfilter sludge, and molten calcium chloride at a temperature within therange of about 700 to 800 C., preferably under an inert atmosphere. Theorder in which the ingredients are introduced into the alloying mixtureis not critical. For example, all of the components can be charged tothe reaction vessel initially. Alternatively, the aluminum may becharged and melted, covered with a layer of calcium chloride, afterwhich the sludge is charged. It is also possible to add the aluminumlast to a molten mixture of sludge and calcium chloride.

As the molten mixture is being stirred, calcium and sodium from thesludge dissolve in the aluminum. The amount of calcium which candissolve, to form a liquid alloy with aluminum, is considerable. On theother hand, sodium is soluble in aluminum only to the extent of about0.01% by weight. However, an important feature of this invention is theincreased solubility of sodium in the aluminum alloy brought about bythe presence of calcium in the aluminum alloy. Sodium solubility isincreased from a level of about 0.01% by weight to a level of up toabout 1.0% by weight. The remaining undissolved sodium in turn reacts toa high degree with the calcium chloride to introduce additional calciuminto the calcium-sodiumaluminum alloy in accordance with the followingequation:

The resulting admixture of CaCl and NaCl acts as a fiu-x to remove theoxides and other inerts which are present in the starting materials. Thecaloium-sodium-aluminum alloy is separated from the flux by conventionalmethods such as bottom tapping from a separated two phase (salt-metalmixture) or by solidification and physically separating the metal layer.Either cobbing or solution of the salts in water may be used.

a sludge of known composition and an excess of calcium chloride inrelation to the sodium, it can be assumed that substantially all of thecalcium in the sludge and about 0.5 wt. percent of the sodium willdissolve in the aluminum and that the rest of the sodium will react withthe calcium chloride to produce calcium.

As described above, a method has been developed whereby increasedamounts of sodium, derived from a low-cost source (sodium cellfilter-sludge), can be introduced into aluminum-calcium master alloys.In addition, sodium from the same low-cost sodium can be used in thesame process to generate calcium from calcium chloride. This methodcontrasts with the use of relatively high cost sodium or calcium whenintroduced as the pure metals.

The invention will be more fully understood by reference to thefollowing illustrative embodiments.

Example I-Preparati0n of master alloy (A) A charge of 40 parts of purealuminum, 5 parts of metallic sodium and parts of anhydrous calciumchloride was placed in a gray cast iron pot equipped with a steel toptightly fitted and of sufiicient height so that the alloy could bepoured into the top for cooling into an ingot by tilting the top belowthe horizontal so that the alloy would flow into the top section. Thismade for easy removal and separation as the upper salt layer and lowermetal layer could be parted easily after cooling. The pot was equippedwith a stirrer to effect good contact between sodium, salt and aluminumin the molten state. For melting the pot was placed in a tilting,induction heated furnace with the cool top protruding out of the furnaceor heating zone. The closed charged vessel was purged of air with argonprior to heating and maintained under argon blanket for the duration ofthe run. Temperature was raised to 800 C. and the contents allowed tomelt. Then rapid stirring was maintained for 15 minutes. The alloy wasallowed to settle for 10 minutes after which the furnace was tilted sothat the contents could flow into the cooler upper section. Afterseparation from salt the crude ingot was washed with water, dried,remelted and cast into 1 /2 inch alloy bars. Results of two separatepreparations are set forth in Table I in runs 2 and 4.

(B) In an exactly similar manner a charge consisting of 42 parts of purealuminum, 10 parts of sodium cell filter sludge (analyzing 20% Ca; Na;20% oxides and inerts) and 40 parts of anhydrous calcium chloride wasput through the heating, agitation and casting cycle. The resultingalloy from several such preparations are shown in Table I, runs 1, 3,and 5. Recovery of metal values in the sludge as calcium plus sodium inthe aluminum was 96%. The conversion of CaCl to NaCl approached 30%.

That aluminum is not functioning as a reducing agent for CaCl underthese conditions is shown by the fact that no aluminum salts appear inthe molten salt layer.

It is also possible, although less economical, to carry out theaforedescribed process using metallic sodium in place of sodium cellfilter sludge as a source of sodium.

The amount of sodium sludge used in proportion to the aluminum informing the calcium-sodium-aluminum alloy varies with the composition ofthe sludge and the composiiton desired of the master alloy. In general,using Example IIM0dificatf0n of aluminum-silicon alloys An untreatedaluminum-6% silicon alloy was used to produce a block casting. As istypical of this type of casting, the surface of the cast block pulls infrom the mold surface where the heat is retained for the longest timescausing about 5 to 10% of the cast blocks to be rejected.

In commercial practice this effect is avoided to some extent by addingchilling irons and extra risers to the mold, which undesirably increasethe labor expenses both before and after the casting is poured.

In accordance with the present invention this undesirable etfect isprevented by the presence of about 0.01 to 0.5% CaNa in the aluminumcasting alloy prior to the pouring step. No extra risers or chillingirons are needed. To eifect this treatment, 044 lb. of a master alloycomprising 4.5% CaNa in aluminum was placed in a hot ladle followed bythe addition of 100 lbs. of molten aluminum-6% silicon alloy. Afterallowing about two minutes for the usual mixing and skimming operations,the treated casting alloy was poured into sand molds. A refinement inthe silicon structure was noted, and a photomicrograph revealed thepresence of much smaller silicon crystals. The improvement in tensilestrength and hardness properties is shown in Table II.

Example llI-M0dificati0n of aluminunnmagnesium alloys To 100 lbs. of analuminum-6% magnesium casting alloy was added 0.05% by weight of calciumplus sodium in the form of a 4.5% calcium-0.2% sodium-aluminum masteralloy. This was accomplished by drawing 100 lbs. of the molten alloyinto a ladle, and then adding 1 lb. of the master alloy in one chunk tothe surface and submerging it into the molten aluminum by aphosphoriziug iron. The master alloy dissolved in less than 1.5 minutes,and the treated aluminum-magnesium alloy was then cast into steel molds.A minimal amount of shrinkage was noted in the casted block aftercooling. In contrast, an untreated aluminum-6% magnesium casting alloygave a cast block product having marked surface shrinkage.

Example IVM0dification 0f aluminum-zinc alloys A casting alloy ofaluminum-7% zinc was treated in the same manner as described in ExampleIII. Approximately 0.512 lb. of a master alloy comprising 7.8%calcium-0.2% sodium-aluminum was combined with 100 lbs. of the castingalloy to give a concentration of 0.04% by weight calcium plus sodium inthe aluminum-zinc alloy. The resulting metal composition was then pouredinto wheel molds to produce castings which have a thin walled hollowsection between the hub and the rim. This thin walled section must holdgrease under pressure during service. Castings made with thealuminum-zinc treated as described above showed substantially noporosity and held grease during service. The use of an untreatedaluminum-zinc alloy, on the other hand, resulted in castings which werevery coarsely porous with a tendency to leak grease. In Table III theimprovement in physical properties achieved by treating thealuminum-zinc alloy by the method of this invention is demonstrated.

The above data show that the method of this invention whereby aluminumcasting alloys are improved by the addition of master alloys containinga combination of an alkaline earth metal and an alkali metal withaluminum. The improvement in surface phenomena achieved by practicingthe present invention is not completely understood. The above dataindicate that it may be separate from the refining of the siliconcrystal structure, since the effect was also achieved on the surfaces ofnon-silicon containing aluminum castings. One possible factor may bebetter contact of the metal or metal alloy with the mold wall resultingin better casting wall-strength by faster cooling. Another possiblefactor may be the existence of a chemical gettering of gases between thewall of the mold and the hot soft metal. At any rate, the aluminum alloycasting surface is a more accurate replica of the mold surface if theformer has been pretreated with a master alloy as described above.Untreated castings are shinier as if they were cast against a gas film,while the treated castings surfaces are a replica of the matte or finesand finish of the interior of the mold.

Although the present invention has been illustrated by the foregoingspecific embodiments, it will be understood that the invention may besubjected to variations and modifications without departing from itsbroader concepts.

What is claimed is:

1. A method for modifying the physical properties of aluminum castingalloys which comprises adding thereto a property-improving amount of analkaline earth metal and an alkali metal in the form of a master alloyconsisting essentially of about 0.5 to 20% by weight of an alkalineearth metal, about 0.01 to 1% by weight of an alkali metal, and about 79to 99% by weight of aluminum.

2. The method of claim 1 wherein is an aluminum-silicon alloy.

3. The method of claim 1 wherein said casting alloy is analuminum-magnesium alloy.

4. The method of claim 1 wherein said casting alloy is an aluminum-zincalloy.

5. The method of claim 1 wherein said alkaline earth metal is calciumand said alkali metal is sodium.

said casting alloy References Cited UNITED STATES PATENTS 3,265,4928/1966 Juckniess -135 0 2,087,269 7/ 1937 Stroup 75-138 DAVID L. RECK,Primary Examiner. RICHARD O. DEAN, Examiner.

1. A METHOD FOR MODIFYING THE PHYSICAL PROPERTIES OF ALUMINUM CASTINGALLOYS WHICH COMPRISES ADDING THERETO A PROPERTY-IMPROVING AMOUNT OF ANALKALINE EARTH METAL AND AN ALKALI METAL IN THE FORM OF A MATER ALLOYCONSISTING ESSENTIALLY OF ABOUT 0.5 TO 20% BY WEIGHT OF AN ALKALINEEARTH METAL, ABOUT 0.01 TO 1% BY WEIGHT OF AN ALKALI METAL, AND ABOUT 79TO 99% BY WEIGHT OF ALUMINUM.